In the field of extruding complex shapes one of the more challenging items to produce is the flexible gaskets used for sealing refrigerators, automobile doors, hatches, and the like. These products utilize a complex cross section which requires considerable dexterity for the die to accurately reproduce the product. In order to accurately extrude these shapes, the die must completely fill while maintaining a continuous flow of plastic. The dies used in this type of process are generally referred to as profile dies. In manufacturing such dies it is often necessary to construct the die, use the die to see how it works and then, through a series of corrections, gradually bring the die into tolerance. This trial and error method is time consuming and expensive.
It is a purpose of this invention to construct a profile die which provides a balanced flow to the extremities of the die passage and to provide a more accurate and simpler way to construct a profile die within closer tolerances. A die with close tolerances will allow the product to be extruded with less material with a resulting savings in cost.
A profile die system is constructed for extruding a flexible product having a complex cross section. The die system of this invention consists of an assembly of axially aligned generally cylindrical modules. A series of passages are provided to deliver molten plastic from the input to the die in a balanced flow. To accomplish this a distribution module is constructed with an inlet to receive plastic from an extruder. The distribution module has a plurality of distribution channels extending downstream in the die system. The distribution channels are sized and positioned to supply selected regions of the extrusion passage. The distribution channels exit at the downstream face of the distribution module. A transition module is positioned downstream of the distribution module and is constructed with a series of transition passages extending from its upstream to its downstream face. In the assembled position, the inlets of the transition passages are aligned with the exits of the distribution channels to receive molten plastic therefrom. The cross section of the transition passages gradually converts a generally cylindrical flow to a cross sectional shape representative of the region of the die selected for the particular passage.
In constructing the distribution channels, it is first necessary to analyze the shape of the extrusion passage and divide its cross sectional area into regions for concentration of plastic according to the nuances of its shape. The number of distribution channels is determined by the number of regions selected. The cross sectional area of each of the distribution channels is designed to be less than the cross section of the profile die system inlet and consistent with the flow area of the region serviced by the channel. In this manner the overall flow volume from the extruder is metered proportionally into the selected regions. Each of the transition passages are constructed to have a reduced cross sectional area from that of the distribution channel with which it communicates. Therefore, the flow volumes in each of the regional flow streams is gradually reduced from the inlet of the distribution module to the extrusion die, thereby creating an overall funnel effect in each of the regional streams. In the transition module the passage is constructed to convert the generally cylindrical flow to a shaped flow more compatible with the entrance to the die passage in each of the selected regions.